President Donald Trump called on Japan to buy more defensive weapons from the United States in order to protect itself from North Korean missiles. The president claimed that the purchase of ballistic missile defenses would make Japan more safe, and that Japan could shoot down Pyongyang's missiles. The truth is a lot more complicated.

"[Japanese Prime Minister Shinzo Abe] will shoot them out of the sky when he completes the purchase of a lot of military equipment from the United States," Trump stated in Japan. Trump made the remarks in Tokyo in the first leg of his five nation, thirteen day tour of Asia.

Trump described Japanese purchases of American defense equipment as a win-win for both countries. "It's a lot of jobs for us, and a lot of safety for Japan, and other countries that are likewise purchasing military equipment from us," The Los Angeles Times quoted Trump as saying. For his part Abe reportedly agreed, noting that his country already buys a lot of U.S. equipment but that Japan should "enhance our defense capability."

The remarks come on the heels of Trump's comments stating he could not understand how Japan, a nation of "samurai warriors" in his words, did not shoot down North Korean missiles overflying its territory.

Japanese F-15 Eagle flight crews scrambling to their aircraft.

U.S. Air Force photo by Tech. Sgt. Benjamin W. Stratton

Japan, a key U.S. defense ally since 1961, routinely buys the best U.S. military equipment it can afford. There are several reasons for this, one being that U.S. military equipment often is the best in the world. Another reason is to improve interoperability between the two militaries. If the Japanese Maritime Self Defense Forces and the U.S. Navy use the same missiles, radars, and communications gear, it's easier for the two navies, language barrier aside, to work together. Thirdly, it increases Japan's political pull and status in the United States.

In recent years, Japan has purchased the Aegis Combat System for warships, the F-35 Joint Strike Fighter, and Patriot missile system from the United States. Japan's missile defense system, the most advanced in the world after the U.S., consists of the Aegis Combat System and the SM-3 ballistic missile interceptor, the latter jointly developed by the two countries, and Patriot.

In practice, Aegis and SM-3 provide so-called "midcourse defense," intercepting enemy warheads traveling through space after separating from their missile/rocket. Any warheads that get through the midcourse defense are ideally shot down by Patriot PAC-3, a short-range ballistic missile derivative of the original Patriot missile. Together, the two systems provided a layered defense system.

Aegis Ashore facility under construction in Deveselu, Romania, 2015.

U.S. Navy photo by Lt. j.g. Alexander Perrien

Recently, Japan's leaderships have pondered improving the country's defenses with a new system called Aegis Ashore. While Japan's current Aegis missile intercept system is based on its fleet of six Kongo and Atago-class guided missile destroyers, Aegis Ashore is a land-based system that doesn't rely on ships. Properly spaced across Japan, two Aegis Ashore locations equipped with SM-3 interceptors can defend most of the country. Against missiles directly targeted against Japanese territory, the system should be pretty effective. Since it's never been tested in combat, nobody really knows.

Could Japan shoot down North Korean missiles overflying its territory? Probably not. North Korea's missiles were not aimed at Japan but rather at an empty patch of ocean outside Japan's missile defense coverage range. Shooting down a ballistic missile as it whizzes by going somewhere else is typically not achievable. The aiming point for the incoming warhead must typically be within range of the missile defenses for a successful intercept to take place. This also makes it unlikely Aegis Ashore could shoot down overflying missiles.

Japan has made great strides in building its ballistic missile defenses. While Aegis Ashore and similar systems can help Japan in case of an actual missile attack, there's little the country can do to stop overflights, with the possible exception of stationing its destroyer fleet in the middle of the Pacific and hoping it gets lucky. And the Pacific is a big ocean.

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